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278

SILVER ALLOYS

In whatever way the sample is taken, the determinations should always

be executed in duplicate.

1. Determination of the Silver.—The methods most commonly used
' the dry or cupeUation method, Volhard's volumetric method with

are

thiocyanate, and Gay-Lussac's sodium chloride method.

(a) CUPELLATION METHOD. This method is based on the fact that the
noble metals, silver, gold and platinum, are unoxidisable at the highest
temperatures, whilst copper and other metals Usually alloyed to the precious
metals oxidise easily and, if in presence of a certain quantity of lead—which
gives a readily fusible oxide—penetrate by imbibition into the cupel. Thus,
the noble metals are separated in the form of a drop, which, on cooling,
yields a button capable of direct weighing.

Apparatus and reagents, (i) Muffle furnace, either coal or gas, the
latter more easy to manipulate and regulate. In order to protect the
operator from the intense heat of the furnace, the latter is usually placed
•in an adjacent room close to the dividing wall, a small aperture in which
gives access to the orifice of the muffle.

2.  Cupels.   These are capsules having the form of an inverted, trun-
cated cone and made with bone dust carefully powdered, calcined, washed
and pressed in a mould.   A good cupel should absorb its own weight of

lead.

3.  A thermo-electric couple with the corresponding pyrometer volt-
meter, to measure the temperature of the muffle.   The couple is placed
in the muffle so that its extremity is very close to the cupel.

^4. Lead free from silver. That obtained by reducing litharge could
be used but its price is too high. Lead almost entirely free from silver is,
however, sold and is quite suitable ; 20 grams of it should be cupelled as a
check.

Preliminary test. The amount of lead to be used for the cupellation
varies with the silver content of the sample, so that it is necessary to make
a preliminary test. The external characters and a test on the touchstone
are sufficient to a skilled operator. A beginner may make use of a method
which is sometimes employed and which consists in cupelling o-i gram of
the sample with 0-5 gram of lead if the metal is soft and white, with i gram
of lead if it is hard, or with 1-5 gram, if it appears reddish.

The amounts of lead to be used for different degrees of fineness are as
follows:

Degree of fineness
of the alloy.

1,000

950
900
800
700
600
600-0 .

Amount of lead to cupel
i gram of the sample

0-3 gram.

io-o

12-0
14-0
I6-I7

Actual test.   If the fineness is above 800, two samples of i gram each,
and if less than 800, two samples of 0-5 gram each are weighed with thealuminium also contains 0-04-0-12% of nitrogen. The